Questions answered: 4-26-2010

Sherry Seethaler

QUESTION: Why do some volcanoes, such as Mount St. Helens, explode violently, while others, such as currently active volcanoes in Hawaii, behave less destructively?

T. Hewitt

ANSWER: Volcanoes display a multiplicity of eruption styles. The explosiveness of a volcanic eruption depends on the chemical composition of its magma, the magma’s viscosity, and the rate at which it rises to the surface.

Types of volcanic eruptions are typically named after well-known volcanoes with characteristic behaviors, although during a period of activity, a volcano may go through a sequence of eruption types. The most violent eruptions that blast ash high into the atmosphere, such as the 1980 eruption of Mount St. Helens, Washington, and the 1991 eruption of Mount Pinatubo, the Philippines, are called plinian.

Plinian eruptions and other explosive eruptions, including Pelean, in which avalanches of hot gas, ash, lava and rock rush downslope, and Vulcanian, characterized by large clouds of gas and ash, are produced from viscous magma that is high in dissolved gases, especially water vapor. At the other end of the violence scale are Strombolian eruptions, with their intermittent blasts of lava clots, and Hawaiian lava fountains, produced from runnier magma that is lower in dissolved gases.

Dissolved gases in magma influence a volcano’s explosiveness because, as the magma ascends from within the earth, it decompresses. The dissolved gases form bubbles, behaving similar to the way soda does when the lid is removed from the bottle. Viscosity of magma influences explosiveness because if magma is thick and syrupy, the bubbles less readily escape from it.

The speed of ascent of magma determines the rate at which the external pressure on the magma decreases. Once decompression causes the bubbles to expand faster than the liquid films surrounding them can spread, the films rupture and a hot mixture of ash and gases ascends to the volcanic vent.

A volcano’s location plays a role in how explosive it is because the composition of magma is changed as it interacts with overlying rocks. Most volcanoes are associated with the boundaries between shifting plates in the Earth’s crust. The periphery of the Pacific Ocean marks the boundary of several plates, and this highly volcanically active region has earned the moniker “Ring of Fire.”

Rocks in plates being pushed beneath one another are often high in water-rich minerals. They produce magma with high water content. In contrast, the Hawaiian Islands are not on a plate boundary. Their activity is attributed to thermal plumes or hot spots. There, upwelling magma does not interact with water-rich rocks. Thanks to the low water content of the magma, Hawaiian volcanoes tend to be relatively gentle giants.

Sherry Seethaler is a UCSD science writer and educator. Send scientific questions to her at Quest, The San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or e-mail sseethaler@ucsd.edu. Please include your name, city of residence and phone number.